PROJECT SUMMARY Despite understanding of the function of myeloid-derived suppressor cells (MDSCs) in vascular remodeling at- tributed to cancer growth and metastases, it is unknown how MDSCs may contribute to development of pulmo- nary hypertension (PH) in pulmonary interstitial inflammation and fibrosis. The central hypothesis of this appli- cation is that MDSCs promote pulmonary hypertension associated with pulmonary fibrosis through arginase- mediated mechanisms. The scientific premise for this hypothesis has been formulated on the basis of preliminary data demonstrating that MDSCs are increased in two models of myeloid-cell stimulation ? clodronate liposome administration and transgenic mice expressing diphtheria toxin receptor on LysM.Cre-expressing cells. Further- more, an increase in lung MDSCs is associated with development of PH in the bleomycin-induced pulmonary fibrosis model, despite a benefit in overall lung fibrosis, and this increase in pulmonary pressure associated with the model is negated by MDSC depletion. Finally, nonspecific arginase inhibition protects against the develop- ment of PH. Guided by these preliminary data, this hypothesis will be examined by pursuing two specific aims: (1) To determine the role of G-MDSCs in pulmonary vascular remodeling in PH associated with bleomycin- induced pulmonary fibrosis; and (2) Examine the contribution of arginase-1 production by MDSCs in develop- ment of PH secondary to bleomycin-induced pulmonary fibrosis. Under the first aim, wild type mice treated with clodronate liposomes, or transgenic mice with inducible macrophage depletion, will be used to determine the effect of an increase in MDSCs on the development of PH in the bleomycin-induced pulmonary fibrosis. Addi- tionally, the effect of MDSC-specific depletion, by anti-Gr1 antibody injection, on progression of PH will be stud- ied. In order to establish the intrinsic functionality of these immune cells, the effect on PH and pulmonary fibrosis of adoptive transfer of MDSCs in the bleomycin model will also be examined. Under the second aim, the role of arginine metabolism will be defined in our model through provision of the substrate, L-arginine; or, testing the contribution of inducible nitric oxide synthase (iNOS) to the development of PH, through either inhibition of the protein with L-NMMA or treatment of mice with global deletion of iNOS, in the bleomycin and clodronate liposome model. Additionally, PH and fibrosis will be assessed in mice treated with arginase inhibitor (nor-NOHA) or those with myeloid-specific depletion of arginase-1. The approach is innovative, because it shifts focus from vasodilator therapy to a fine-tuned immunoregulatory mediator, MDSCs, in the pathobiology of PH. The approach is signifi- cant, because it is expected to have application in establishing grounds for study of immunobiological drugs targeting MDSCs in the treatment of patients with PH associated with chronic lung disease.